Give Vibratory Screens A Fair Shake

Understand the key factors in their selection and use

By Amin Almasi, mechanical consultant

Plants rely on vibrating screens for a variety of duties such as separating, sifting, sorting, dewatering and classifying. Some vibrating screens, if designed properly — i.e., with a full range of adjustments, even can serve as feeders. Today’s devices generally boast higher efficiency, lower energy consumption and better reliability than traditional ones, thanks to improved design techniques and accumulated knowledge and know-how. However, success in an application still depends upon proper selection of screens, vibrating mechanisms and auxiliaries.

Many chemical processes require control of the size characteristics of a feed material. The vibrating screen is the most widely used device to split a feed of particulate materials into different grades of coarse and fine materials. Depending upon the degree of material separation and classification required by an application, designs may include two or three decks. In many units, oversize material goes to size reduction equipment for processing to an acceptable size. Even the most efficient vibrating screen will retain some undersize material.

Design, Sizing And Selection

A vibrating screen consists of many different components, e.g., a frame, vibrating mechanism, springs, screen deck(s), liners, etc. Six factors — width, length, screen inclination angle, vibration frequency, vibration amplitude and vibration pattern — are important in the design and operation of vibrating screens. Two parameters — capacity and efficiency — usually define the performance of any vibrating screen. These performance parameters aren’t independent; efficiency usually varies inversely with capacity. The screen capacity closely relates to the width. The screen length mainly affects the screening efficiency; the efficiency generally increases with length. The efficiency also depends, to a lesser degree, on the inclination angle, with efficiency declining as the angle gets steeper.

Efficiency could reach up to ≈95% using a screen with large dimensions and the best technologies for everything. This rarely has been attained and requires dedicated and costly tests. In practice, efficiencies of 80–90% are more realistic when using optimized designs. Many modern, properly designed vibrating screens achieve efficiencies of ≈85–88%.

Length-to-width ratio typically is 2.5:1 to 3.5:1.It’s not common to achieve efficiencies greater than 80% using a relatively short screen (say, with a ratio of ≤2:1) or angles >30°. Many ordinary and low-cost vibrating screens only attain efficiencies of ≈65–80%, even though their manufacturers claim high efficiencies. Therefore, it’s essential to properly evaluate vibrating screen designs and verify claimed performance and parameters.

Inclined screens commonly are used. Many modern vibrating screens rely on an angle between 10° and 25°. In many inclined screens, a single unbalance, rotating on a horizontal axis, generates a simple motion (circular or elliptical) mainly in the vertical plane. This motion imparts little positive movement to particles. Movement mainly stems from the screen inclination and the force of gravity, which cause the particle mass to travel at velocities of ≈0.3–0.6 m/s.

Angles lower than 10° aren’t common in conventional vibrating screens. Some specially designed linear-stroke vibrations can allow placing the vibrating screen at shallow angles (say, <10°). However, conventional screens with a circular or elliptical type of vibratory motion often require >15° angles. Screens set at a shallow angle or near horizontal usually employ a pair of unbalances, rotating in opposite directions on parallel horizontal axes, to generate a (nearly) straight-line reciprocating motion, inclined to the plane of the screen surface at 40°–50°. Velocities on a horizontal surface typically run ≈0.3–0.5 m/s but can be increased if necessary by inclining the screen downward (say, ≈10–15°). On the other hand, steep angles (>35°) aren’t common. Always check that a design allows convenient slope adjustment.

Vibration screens come in standard models as well as custom-engineered versions. In standard models, the frame and other major components are fixed; only the screen and a few other items are tailored for each application. This enables relatively fast delivery. In contrast, custom-engineered units are expensive and incur long lead times, making them sensible only for special situations.

Although vibrating screens have been widely used for many decades, engineers and operators at chemical plants often possess only limited knowledge of their design, installation and operation. Generally, the screening operation is complex. Theoretical models offer little practical utility. So, instead, for design, selection and sizing, users typically rely on empirical curves and formulae provided by manufacturers in conjunction with information on the feed rate, particle sizes, etc., to determine the type of screen deck and details such as the width and length of the screen, screen material, aperture size and percentage open area. Each major manufacturer has developed its own set of curves and formulae. However, many textbooks and handbooks present the curves and formulae from the same well-known manufacturer.

Operational flexibility is important to deal with changes in feed materials or processing conditions as well as emergency situations. For instance, proper operational parameters can enable screening of different feed materials such as sticky and adhesive bulk solids by utilizing automatic and repetitive pulsing vibrations.

Besides classification, another well-known application of vibrating screens is for dewatering. This can involve two different tasks: separating free water and removing surface moisture from the wet materials. The rate of dewatering usually depends on the instantaneous moisture content of wet materials as well as the amplitude of the vibration. In general, a vibrating screen with larger vibration amplitudes provides better dewatering performance.

Factors Affecting Vibratory Motion

Vibrating screens are characterized by dynamic motions mostly in the vertical plane (although many different patterns have been used for vibration of screens). The acceleration of such movements typically ranges from 3 to 6 g or even more. The lifting and dropping effect expands the effective screening bed. Pure vertical motion gives individual particles limited opportunity for finding and passing through an opening. Added horizontal motion can resolve this issue and spread materials over the screen. On the other hand, the vertical force component acts to eject near-size particles stuck in the openings, thus resisting progressive blinding, and the turbulent expansion of the material bed prevents packing. Therefore, effective screening usually requires a proper combination of vertical and horizontal vibration.

The performance of a vibrating screen can be optimized for any application by changing the vibration amplitude and frequency. The screening rate and performance respond more to changes in amplitude than frequency, although higher frequencies are useful to increase resistance to near-size blinding, etc. As a general rule, the amplitude should go up if particle size or bed depth increases, with frequency adjusted to maintain peak acceleration in the required range (often, as noted, 3–6 g, and, most commonly, ≈4.5–6 g, for many applications).

As working capacity (feed rate) is increased, the vibration amplitude also should increase. However, the optimum depends on the particular application. Generally, as feed rate goes up, efficiency declines even for optimized designs.

The strength of the screen and equipment imposes limits on vibration amplitude but you generally can vary frequency (say, up to ≈50 Hz). A good recommendation is to use variable speed options for the screen vibratory excitation to better optimize operation. In some very special designs, where the screen is delicate and vibration amplitude is seriously limited, raising the frequency and using a relatively steep inclination angle, say, even 35–40°, may partially compensate.

The mass of material handled by a unit always varies somewhat; this causes a change in vibration characteristic, specifically, natural frequencies and vibration amplitudes. The situation is far more complicated than ones in simplified vibration models. Vibrating screens come in three dynamic categories: pre-resonant, resonant and super-resonant. Many screens in practice operate as pre-resonant or super-resonant systems. An important consideration is to minimize the variation in the vibration amplitude; considering chaotic changes in mass and other dynamic characteristics that can result in amplitude changes if equipment works at resonance, operation at resonance generally isn’t recommended.

Vibrating screens usually are supported on compression springs, either steel coils, rubber or pneumatic. Fixed at one end, the other end should follow the motion of the vibrating system. The resistance of the spring to displacement in both vertical and horizontal directions determines the amount and direction of force transmitted through the spring to the supporting foundation or framing. The static load supported by each spring is simply the weight of the vibrating system divided by the number of springs; the static deflection is the same weight divided by the total stiffness. Steel coil springs find wide use with vibration screens; they present linear behavior. For reasons of stability, the steel coil springs’ maximum static deflection generally is limited to ≈15 mm, with lower values, say, ≤9 mm, most commonly selected. Rubber springs have a nonlinear load/deflection curve, so they only are used in special designs. Pneumatic springs can offer nearly linear behavior but also only suit special applications. Stresses resulting from transmitted vibrations, superimposed on static stresses due to static loading, can cause fatigue failures in the spring system or structural connections.

Operation, Reliability And Maintenance

The basic operation of a vibrating screen is simple. The screen presents a barrier to the passage of oversize material but passes undersize material. Vibration facilitates this process by creating the movement needed to ensure that each particle has opportunities to reach the screen. In fact, each particle should receive several opportunities to pass through the screen. However, the actual screening process isn’t quite that simple. It involves many side effects and complicated sub-activities; all of these require very careful review and consideration. For instance, the motion imparted to solid particles during screening can generate dust. It also can result in the particles becoming electrostatically charged. Static charges can lead to screen blinding and significantly reduce the efficiency of the screen. In general, blinding is a major issue with any screen. In addition, static charges can act as an ignition source, which can be a major risk for some applications. All the metal components of the screen system, e.g., screens, frames, etc., require proper grounding and bonding. This will remove the charges from the system itself but a residual charge may persist on the solids. Handling materials such as ignitable particulate solids demands more care. For example, you must limit relative velocities caused by the movement (such as vibration, shaking, oscillation, etc.); as a rough guide, they should be <1 m/s.

Loads on a vibrating screen or hard connections to upstream or downstream equipment can affect the natural frequencies of the system and its vibration. As examples, hoppers with their contents, ducts, other equipment, etc., often are supported by the vibrating screen frame — and can impact system performance.

Vibratory screens frequently use an isolation system or a flexible connection to minimize the forces transmitted to the surrounding equipment and structures. The benefit of flexible connections does come at a price, though, because they are weak points in the system for reliability. Design of flexible connections and evaluation of their reliability and life requires care.

Reliability and maintenance also are important issues for the vibratory screen itself, of course. Along with regular exposure to the erosion, corrosion and other abuses from materials handled, vibrating screens experience high vibration and heavy dynamic loads. Many other adverse effects such as those from dust generation, etc., also can afflict the device. So, not surprisingly, frequent component failures have been reported.

Material and design details of screens need adequate attention. Strong construction is essential; all components subject to particularly severe stresses or dynamic loads require very careful design and fabrication. Bearing issues have been a major problem. Proper bearing selection and sizing is crucial. In addition, the bearing should be installed away from the working areas of the equipment and far from handled materials — and kept dust free.

Finally, ensure that screen changing is easy and straightforward, and that deck clamping and tensioning arrangements allow for convenient, reliable and safe screen deck maintenance. Use abrasion-resistant and readily replaceable liner pieces to protect screen structural elements.


 

AMIN ALMASI is a mechanical consultant based in Sydney, Australia. Email him at amin.almasi@ymail.com.